Mammalia: Visceral Systems 



611 



\ \ PalatineJ 



Larynx 



Esophagus 



Fig. 469. Alimentary and respiratory passages of a mammal. (Compare with 



Fig. 435.) 



As human mammals well know, it is a place where distressing accidents 

 may occur. The fact that most mammals break up the food more or 

 less, or even chew it finely (as in the cud-chewing herbivores), increases 

 the danger of food entering the trachea. 



Crocodilians, in adaptation to aquatic living, have acquired a 

 secondary palate whereby the nasal passages are prolonged backward 

 to the close vicinity of the glottis. From the dorsal surface of the 

 posterior region of the tongue, a transverse fold of the skinlike outer 

 layer of the tongue extends upward to meet a similar fold which 

 descends from the under surface of the bony palate. These folds con- 

 stitute a valve which can be opened to allow food to pass back into 

 the esophagus, but may be closed to prevent water from passing into 

 the nasal cavities and trachea. The nostrils are on the dorsal surface of 

 the snout. Therefore an alligator can submerge, leaving only the 

 nostrils exposed to the air, and can meanwhile breathe freely while 

 the month is filled with water and perhaps busy with struggling prey. 

 In general, the anatomy and history of crocodilians indicate that they 

 are much more closely allied to birds than to mammals. The existence 

 of very similar palatal and pharyngeal arrangements in crocodilians 

 and mammals is therefore probably to be regarded as a striking case of 

 convergence in evolution. 



In mammals the bony secondary palate comes somewhat short 

 of reaching the glottis. Its deficiency is compensated for by addition 

 of a "soft palate" which arises from the posterior edge of the bony 

 palate and extends back to the region of the glottis. This soft palate 



